The 3' end of 16S rRNA in 30S subunits can be labeled with 32PCP using RNA ligase. Treatment of the ribosomes with RNase T1 releases 3' end-labeled fragments which can be separated by gel electrophoresis and identified. Differences in oligonucleotide patterns are noted between active and inactive 30S subunits. The effects of protein S1 and 50S subunits are now being investigated. The E. coli 50S ribosomal subunit contains four copies of protein L7/12. Under appropriate conditions two or all four of these proteins can be removed from the subunit. The subunits with 0, 2 and 4 copies of L7/12 can be separated into three distinct bands by electrophoresis in agarose-acrylamide composite gels. The structural and functional roles of this protein are now being examined in subunits with 0, 2, 4 copies. The successful electrophoretic separation of yeast polyribosomes in composite gels has permitted us to study the role of specific antibiotics on translation and, in particular, ribosome and polyribosome structure. Cyclohexamide, as an example, blocks binding of 60S subunits to 40S initiation complexes on polyribosomes. These "halfmer" structures are now being examined with respect of initiation factor content and as a "substrate" for assaying factor mediated binding of 60S subunits to initiation complexes.